I think bmxbumpkin is thinking of the belief that invaders don't breed (I've seen it said in the past, and I haven't heard of anyone saying they've seen it happen).
Residents, at least, can breed (I've managed to get both parties of a married goblin couple to petition and produce a baby).
FantasticDorf's proposal is good in theory, but I doubt it works well in practice. I think to get a reasonable chance of producing offspring you really need to catch both parties of married couples. There are two reasons for my conclusion:
1. Only a fairly small percentage of visitors are unmarried, and a large percentage of those won't marry. For some reason performance troupe members seem to be more likely to be unmarried but willing than others.
2. I assume the same age restrictions for marriage that apply for dwarves are applied to other races as well, i.e. a +/- 10 year age span. Elves and goblins are "immortal", so you need a rather large pool to get matches. Humans, with their short life spans, are somewhat easier, but still, you need a lot of them, and point 1 makes getting a lot of them tricky.
I've seen the militia training hookup advice before, but it has worked extremely poorly for me. My experience is that militia members get very few acquaintances, and fellow militia members do not seem to be more likely than others. In fact, random visitors seem to soak up almost all "relation points", so citizens can live for years in the fortress with two pages of visitors listed on the relations list and only a rather small number of citizens.
Pre honeymoon suites have worked fairly well for me, though, PROVIDED at least one of them has the other on their relations list (and other pre conditions are fulfilled, of course).